This invention relates to electrical energy reduction means for controlling artificial light intensity in accordance with the level of available natural light, and more particularly relates to an energy saving circuit and system of this type which requires only a single user adjustment and which reduces unnecessary cycling to a minimum.
It is well known that in areas which are illuminated by both natural light and artificial light that the artificial light can be reduced when the level of natural light in the area is sufficient to serve predetermined purposes.
A simple photosensor which switches an electromechanical relay in accordance with the level of natural illumination is commonly used to control exterior lighting, such as lighting in parking lots or building floodlights. While the simple arrangement of a photosensor and an electromechanical relay performs satisfactorily for control of an exterior light, such a simple arrangement cannot be used for control of indoor lighting. This is because the control of light level in indoor lighting situations is much more critical than for an outdoor arrangement, so that the device may continually switch indoor lamps on and off to adjust for relatively small changes in outdoor lighting due to clouds obscuring the sun, and the like. This would not cause the switching of the device in an outdoor installation where the adjustment is relatively non-critical and artificial lights are not turned on until substantial dark conditions are achieved.
U.S. Pat. No. 4,281,365, in the names of Elving and Carlson, discloses a control arrangement which can be used indoors. A comparator circuit is employed which receives inputs from a photoelectric sensing means and from an input reference circuit. A hysteresis circuit is also disclosed so that the light level at which the comparator output level goes from a high state to a low state is significantly different from the light level at which the output goes from a low state to a high state. The comparator output then actuates a timing means which delays the operation of a power control device which controls the indoor lamps and delays their change in state for some fixed period after the change in comparator output state occurs. This time delay is intended to prevent frequent on/off switching cycles due to broken cloud cover which causes frequent changes in the natural light level.
In a system of this type, however, no matter what time delay period is chosen for the operation of the system, rapid variation of natural daylight conditions can still cause sufficiently frequent cycling to result in extremely uncomfortable work conditions for those within the controlled area. Furthermore, several separate adjustments must be made in setting the system operation. Thus, there must be separately adjusted the switch-on level, the switch-off level (or amount of hysteresis), the time delay for turning lights on after light has increased above a given threshold value and the time delay for beginning to reduce light level after a lower threshold has been reached. These adjustments are generally made by potentiometers or the like and each single adjustment may affect all of the others. Consequently, adjustment becomes a difficult and tedious task and takes a great deal of time since one must observe the system operation over widely varying ranges of natural light and conditions and the person making the adjustment must have a high level of understanding of the system operation. As a result of these difficulties, it has been found that in actual installations the system is never satisfactorily adjusted, so that the potential advantages of the system are unavailable to the average user and the potential energy savings are lost.